Prospects for studies of the free fall and gravitational quantum states of antimatter

TL;DR

This paper reviews ongoing experiments and prospects for observing gravitational quantum states of antimatter, focusing on antihydrogen and positronium, to deepen understanding of antimatter gravity interactions.

Contribution

It discusses the potential to observe gravitational quantum states of antimatter, extending techniques from neutron studies to antimatter systems like antihydrogen and positronium.

Findings

Antihydrogen can be used to measure gravitational free fall with high precision.

Quantum reflection enables filtering schemes to improve measurement accuracy.

Positronium may also exhibit gravitational quantum states in future experiments.

Abstract

Different experiments are ongoing to measure the effect of gravity on cold neutral antimatter atoms such as positronium, muonium and antihydrogen. Among those, the project GBAR in CERN aims to measure precisely the gravitational fall of ultracold antihydrogen atoms. In the ultracold regime, the interaction of antihydrogen atoms with a surface is governed by the phenomenon of quantum reflection which results in bouncing of antihydrogen atoms on matter surfaces. This allows the application of a filtering scheme to increase the precision of the free fall measurement. In the ultimate limit of smallest vertical velocities, antihydrogen atoms are settled in gravitational quantum states in close analogy to ultracold neutrons (UCNs). Positronium is another neutral system involving antimatter for which free fall under gravity is currently being investigated at UCL. Building on the experimental techniques under development for the free fall measurement, gravitational quantum states could also be observed in positronium. In this contribution, we review the status of the ongoing experiments and discuss the prospects of observing gravitational quantum states of antimatter and their implications.